Surgically Implantable Knee Prosthesis

ABSTRACT

A prosthesis is provided for implantation into a knee joint compartment between a femoral condyle and its corresponding tibial plateau. The prosthesis is configured to be translatable with respect to the tibial plateau during knee articulation, and includes a body having a generally elliptical shape in plan and a pair of opposed surfaces including a substantially flat bottom surface and an opposed top surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/831,009, filed Apr. 22, 2004, which is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a prosthetic device which issurgically implantable into a body joint, and more particularly to aknee joint prosthesis which may be surgically implanted between thefemoral condyle and tibial plateau of the knee compartment.

2. Background Art

Articular cartilage and meniscal cartilage provide the mobile weightbearing surfaces of the knee joint. Damage to these surfaces isgenerally due to genetic predisposition, trauma, and/or aging. Theresult is usually the development of chondromalacia, thinning andsoftening of the articular cartilage, and degenerative tearing of themeniscal cartilage. Various methods of treatment are available to treatthese disease processes. Each option usually has specific indicationsand is accompanied by a list of benefits and deficiencies that may becompared to other options.

The healthy knee joint has a balanced amount of joint cartilage acrossthe four surfaces of this bicompartmental joint (medial femoral condyle,medial tibial plateau, lateral femoral condyle, and lateral tibialplateau). In patients with osteoarthritis, the degenerative processtypically leads to an asymmetric wear pattern that leaves onecompartment with significantly less articular cartilage covering theweight bearing areas of the tibia and femur than the other compartment.Most commonly, the medial compartment of the knee joint is affected moreoften than the lateral compartment.

As the disease progresses, large amounts of articular cartilage are wornaway. Due to the asymmetric nature of the erosion, the alignment of themechanical axis of rotation of the femur relative to the tibia becomestilted down towards the compartment which is suffering the majority ofthe erosion. The result is a varus (bow-legged) deformity in the case ofa medial compartment disease predominance, or a valgus (knock-kneed)deformity in the case of lateral compartment disease predominance.Factors such as excessive body weight, previous traumatic injury, kneeinstability, the absence of the meniscus, and genetic predisposition allaffect the rate of the disease.

Osteoarthritis is usually defined in stages of Grade I through V, withGrade III revealing significant articular cartilage loss, Grade IVrevealing some eburnation of the subchondral bone, and Grade V detailingboth significant articular loss and bone loss. The disease manifestsitself as periodic to continuous pain that can be quite uncomfortablefor the patient. The cause of this pain is subject to many opinions butit is apparent that, as the joint compartment collapses, the collateralligament on the side of the predominant disease becomes increasinglyslack and the tibial and femoral axes move, for example, from a varus toa valgus condition. This increases the stress on the opposing collateralligament as well as the cruciate ligaments, and shifts the load bearingfunction of this bicompartmental joint increasingly towards the diseasedside. This increasing joint laxity is suspected of causing some of thepain one feels. In addition, as the bearing loads are shifted, the bodyresponds to the increased loading on the diseased compartment with anincreased production of bony surface area (osteophytes) in an attempt toreduce the area unit loading. All of this shifting of the knee componentgeometry causes a misalignment of the mechanical axis of the joint. Thismisalignment causes an increase in the rate of degenerative change tothe diseased joint surfaces, causing an ever-increasing amount ofcartilage debris to build up in the joint, and further causing jointinflammation and subsequent pain.

Currently, there is a void in options used to treat the relatively youngpatient with moderate to severe chondromalacia involving mainly onecompartment of the knee. Current treatments include NSAIDS, cortisoneinjections, hyaluronic acid (HA) injections, and arthroscopicdebridement. Some patients cannot tolerate or do not want the risk ofpotential side effects of NSAIDS. Repeated cortisone injections actuallyweaken articular cartilage after a long period of time. HA has shownpromising results, but is only a short term solution for pain.Arthroscopic debridement alone frequently does not provide long lastingrelief of symptoms.

Unfortunately, the lack of long term success of these treatments leadsto more invasive treatment methods. Osteochondral allografts andmicrofracture techniques are indicated for small cartilage defects thatare typically the result of trauma. These procedures are not suitablefor addressing large areas of degeneration. In addition, osteochondralallografts can only be used to address defects on the femoral condyle,as tibial degeneration cannot be addressed with this technique. Hightibial osteotomy (HTO) corrects the varus malalignment between the tibiaand the femur but, because it is performed below the joint line, it doesnot fill the cartilage void or re-tension the medial collateral ligament(MCL). Removing bone and changing the joint line does not complicate theconversion to total knee arthroscopy (TKA). However, an HTO does leave ahard sclerotic region of bone which is difficult to penetrate, makingconversion to a total knee replacement technically challenging.

Currently, patients with large tibial defects require replacement of theexisting surfaces with materials other than articular cartilage, namelywith uni-condylar (UKR) or total (TKR) knee replacements. Unfortunately,these procedures resect significant amounts of bone (typically 7-9 mm),and primary (first arthroplasty performed on the joint) procedures havea functional life span of only 5-10 years, such that younger patientswill likely require revision surgery as they age. Furthermore, theamount of bone loss that is inherent in a UKR or TKR makes a revision(secondary) procedure much more difficult in the future as even morebone must be removed. Revision knee replacement surgery is usuallyextensive and results in predictably diminished mechanical lifeexpectancy. Therefore, it is best to delay these types of bone resectingsurgeries as long as possible.

The only true solution is to rebuild the defective joint by “filling”the joint space with more articular bearing material through a completeresurfacing of the existing femoral condyle and tibial plateau. Byreplacing the original cartilage to its pre-diseased depth, the jointmechanical axis alignment is restored to its original condition.Unfortunately, these natural articular materials and surgical technologyrequired to accomplish this replacement task do not yet exist. Thealternative method is to fill the joint space with a spacer thatreplaces the missing articular materials.

Attaching a new bearing surface to the femoral condyle is technicallychallenging and was first attempted, with limited success, over 40 yearsago with the MGH (Massachusetts General Hospital) knee. Like a dentalcrown, it covered both the femoral condyles with Vitallium (CoCr) andwould bear against the existing tibial plateau. Tibial covering devicessuch as the McKeever, Macintosh, and Townley tibial tray maintained theexisting femoral surface as the bearing surface but, like the MGH knee,all required significant bone resection, thus making them less thanideal solutions as well. These devices also made no particular attemptto match the patient's specific femoral or tibial geometry, thusreducing their chances for optimal success. Because these devices weremade of CoCr, which has different viscoelastic and wear properties fromthe natural articular materials, any surface geometry which did notclosely match the bearing surface of the tibia or femur could causepremature wear of the remaining cartilage due to asymmetric loading.

Newer materials technologies in development including filling the jointspace by injecting polyurethane (U.S. Pat. No. 5,795,353) into the jointand anchoring it with holes drilled into the tibial plateau. Othersinclude a series of polymeric materials such as PVA hydrogels in atitanium mesh (see Chang et al, Journal of Biomedical Materials Research37, 51-59, 1997), biodegradable anhydride prepolymers that can becross-linked with irradiation by UV light (U.S. Pat. No. 5,902,599), andin vivo grown articular chondrocytes in a collagen fiber or otherbiocompatible scaffold (U.S. Pat. No. 5,158,574). Other low surfaceenergy materials, such as low temperature isotropic (LTI) pyroliticcarbon, have been investigated as bearing surfaces as well. However,these techniques are limited by one's ability to first of all fashionthese materials in a conformal manner to replicate the existing kneegeometry, while at the same time maintaining their location within thejoint, while further being able to survive the mechanical loadingconditions of the knee.

U.S. Pat. Nos. 6,206,927 and 6,558,421 and co-pending U.S. applicationSer. No. 10/232,608, each of which are incorporated by reference herein,disclose a prosthesis for the knee compartment which fills the jointspace in order to replace the missing articular materials. Thisprosthesis provides an anatomically correct bearing surface for both thetibial plateau and femoral condyle to articulate against. Additionally,the prosthesis reduces the concentrated loads on the femoral condyle andits articular cartilage and maintains proper spatial location of thefemoral condyle to the tibial plateau, thereby restoring normal jointalignment. Advantageously, the prosthesis does not require any boneresection or any means of bone fixation.

In addition to these benefits, it is also desired to provide aunicompartmental prosthesis which accommodates patients havingrelatively large defects on the tibial plateau, defined as bone lossgreater than about 0.5 mm.

SUMMARY OF THE INVENTION

Accordingly, a prosthesis is provided for implantation into a bodyjoint, such as the knee joint compartment between a femoral condyle andits corresponding tibial plateau. The prosthesis includes a hard bodyfree of any means of fixation within the knee joint compartment. Thebody has a generally elliptical shape in plan and has a substantiallyflat bottom surface and an opposed top surface.

In a preferred embodiment, the top surface is generally concave and thebottom surface includes a contour that is generally the same as acorresponding contour of the tibial plateau following any resection. Thebody is preferably at least partially constructed of a biocompatiblematerial such as ceramics, metals, metal alloys, hydrogels, reinforcedand non-reinforced thermoset or thermoplastic polymers, or compositesthereof. The body can include at least one portion made of a relativelylow modulus material, such as reinforced and non-reinforced elastomericpolymers, viscous-elastic materials, and hydrogels. Furthermore, thebody can be at least partially constructed from a material capable ofcontaining living cells. Still further, the body can include anassociated active material, and can include an applied surface coating.

In further accordance with the present invention, a peripheral edgeextending between the top and bottom surfaces and includes a first side,a second side opposite the first side, a first end, and a second endopposite the first end. A first dimension D is defined by the first endand the second end, and a second dimension F is defined by the firstside and the second side, wherein the dimension F is from about 0.25D toabout 1.5D. Both the top and bottom surfaces are preferably contouredsuch that the prosthesis is self-centering within the knee jointcompartment. In one embodiment, one of the first and second sidesincludes an indentation therein. In another embodiment, one of the firstand second sides is generally straight. Outside edges along a peripheryof the body are rounded. A periphery of the body is on average ofgreater thickness than a central portion of the body, and preferably asection across the body from the first end to the second end generallyhas the shape of a negative meniscus. In a preferred embodiment, asection across the body from the first side to the second side has athickness at a periphery of the first side which is larger on averagethan a thickness at a periphery of the second side.

According to the present invention, a prosthesis is provided forimplantation into a body joint. The prosthesis includes a body free ofany means of fixation within the joint and having a generally ellipticalshape in plan. The body includes a substantially flat bottom surface, agenerally concave top surface, and a peripheral edge extending betweenthe top and bottom surfaces. The peripheral edge has a first side, asecond side opposite the first side, a first end, and a second endopposite the first end, where one of the first and second sides includesan indentation therein.

In further accordance with the present invention, a prosthesis isprovided for implantation into a body joint. The prosthesis includes abody free of any means of fixation within the joint, the body having agenerally elliptical shape in plan and a substantially flat bottomsurface and a generally concave top surface. The body includes aperipheral edge extending between the top and bottom surfaces and havinga first side, a second side opposite the first side, a first end, and asecond end opposite the first end, where one of the first and secondsides is generally straight.

Still further, a prosthesis is provided for implantation into a kneejoint compartment between a femoral condyle and its corresponding tibialplateau. The prosthesis includes a body free of any means of fixationwithin the knee joint compartment, the body having a generallyelliptical shape in plan and having a top surface and an opposed,substantially flat bottom surface. The body comprises a material capableof spanning any defects in the femoral condyle and tibial plateauwithout substantially deforming into the defects.

Correspondingly, a method is provided for implantation of a prosthesisinto a knee joint compartment between a femoral condyle and itscorresponding tibial plateau. The method includes providing a prosthesisincluding a hard body free of any means of fixation within the kneejoint compartment, the body having a generally elliptical shape in planand having a top surface and an opposed, substantially flat bottomsurface. The method further includes surgically exposing the knee jointcompartment and inserting the prosthesis into the knee jointcompartment.

The method can further include determining a size and shape of theprosthesis required by examination of the knee joint, where theexamination can include X-ray imaging or MRI imaging. The prosthesis canbe selected from a library of prostheses of standard shapes and sizes,or alternatively a custom prosthesis can be generated where the size andshape are at least partially based on the examination of the knee joint.If necessary, the condition of tissue located in the knee jointcompartment can be altered, which can include resecting a portion of thetibial plateau or the tibial spine. An active material can be associatedwith the body, and a surface coating could be applied, such as for thereduction of friction.

In further accordance with the present invention, a method is providedfor correcting misalignment in an axis of rotation of a knee joint. Themethod includes providing a prosthesis including a hard body free of anymeans of fixation within the knee joint, the body having a generallyelliptical shape in plan and having a top surface and an opposed,substantially flat bottom surface. The method further includessurgically exposing the knee joint, and inserting the prosthesis intothe knee joint to at least partially correct the misalignment of theaxis of rotation. The method can include inserting the prosthesis into amedial compartment of the knee joint and moving the axis to a less varuscondition, or can include inserting the prosthesis into a lateralcompartment of the knee joint and moving the axis to a less valguscondition.

The above features and advantages, along with other features andadvantages of the present invention are readily apparent from thefollowing detailed description of the best mode for carrying out theinvention when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a first embodiment of animplantable knee prosthesis according to the present invention;

FIG. 2 is a top plan view of the prosthesis of FIG. 1;

FIG. 3 is a side elevational view of the prosthesis of FIG. 1;

FIG. 4 is an end elevational view of the prosthesis of FIG. 1;

FIG. 5 is a perspective view illustrating a second embodiment of animplantable knee prosthesis according to the present invention;

FIG. 6 is a top plan view of the prosthesis of FIG. 5;

FIG. 7 is a side elevational view of the prosthesis of FIG. 5;

FIG. 8 is an end elevational view of the prosthesis of FIG. 5;

FIG. 9 is a cross-sectional view of the prosthesis of FIGS. 1-4 takenalong line A-A of FIG. 2;

FIG. 10 is a cross-sectional view of the prosthesis of FIGS. 1-4 takenalong line B-B of FIG. 2;

FIG. 11 is a top plan view of the prosthesis according to the presentinvention with reference to a coordinate system;

FIG. 12 illustrates an exemplary placement of the prosthesis accordingto the present invention in a knee joint; and

FIG. 13 is a perspective view of a prosthesis according to the presentinvention which includes a cusp.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The prosthesis according to the present invention is designed to besurgically implantable into a body joint to replace damaged tissuetherein. More particularly, the prosthesis of the present invention is aunicompartmental device suitable for minimally invasive, surgicalimplantation into a knee compartment requiring little or no boneresection. The knee compartment is defined by the space between afemoral condyle and the respective tibial plateau, in which a portion ofthe natural meniscus is ordinarily located. By effectively replacingworn articular material, the prosthesis of the present inventionrestores the normal joint alignment and provides a smooth bearingsurface against which the femoral condyle can articulate. Degenerationof the femoral anatomy is significantly reduced because the conformingfemoral surface of the prosthesis accommodates the complex shape of thefemoral condyle in extension as well as in flexion. Further, itessentially eliminates articulation of the femoral condyle against thetibial plateau, thereby preventing further degradation of the tibialsurface. By occupying the joint space and retensioning the collateralligaments, the prosthesis according to the present invention improvesjoint stability and restores the limb to a more normal mechanicalalignment.

By the term “unicompartmental” it is meant that each prosthesisaccording to the present invention is suitable for implantation into butone medial or lateral compartment defined by the space between a femoralcondyle and its associated tibial plateau. In other words, the presentprosthesis is not a bicompartmental prosthesis which, in one rigiddevice, could be inserted into both of the two femoral condyle/tibialplateau compartments. In many, if not most, cases a prosthesis will beinserted into one compartment only, either the medial or lateralcompartment. Most often, it will be the medial compartment as themeniscus and associated articular surfaces in the medial compartment aremost subject to wear and damage. Additionally, it is possible to inserttwo separate prostheses into the medial and lateral compartments of thesame knee, or to use two such prostheses that are mechanically, butnon-rigidly, linked. Advantageously, the prosthesis according to thepresent invention functions to at least partially correct misalignmentin the knee axis of rotation due to disease. More specifically, whenplaced in the medial compartment, the prosthesis moves the knee axis 300(see FIG. 12) into a less varus, more valgus condition (typically 0-5°valgus). Likewise, when placed in the lateral compartment, theprosthesis moves the knee axis 300 into a less valgus condition.

The prosthesis according to the present invention is preferablytranslatable but self-centering. By “translatable” it is meant thatduring natural articulation of the knee joint the prosthesis is allowedto move or change its position, such that articulation of the kneeresults in a modest amount of lateral/medial and anterior/posteriortranslation of the prosthesis relative to the tibial plateau. Thus, thepresent prosthesis preferably is devoid of means of physical attachmentwhich limit its movement, for example, screws, mating ridges anddepressions, porous areas to accommodate tissue regrowth, and the like.

The femoral condyle has two major anterior/posterior radii such that,when the knee is in full extension, one radius position is in contactwith the tibial plateau while, during flexion, another portion of thefemoral condyle is in contact with the tibial plateau. In addition, thefemur rotates with respect to the tibia during flexion, thereby changingthe orientation of the femoral anatomy to the tibial plateau. The term“self-centering” means that upon translation from a first position to asecond position during knee articulation, the prosthesis of the presentinvention will return to substantially its original position as thearticulation of the knee joint is reversed and the original kneeposition is reached. Thus, the prosthesis will not progressively “creep”towards one side of the compartment in which it is located, but ratherthe prosthesis is shaped such that the contour of the prosthesis and thenatural articulation of the knee exerts a restoring force on thefree-floating prosthesis. The angle of attack of the femoral condyleand/or tibial plateau bearing surfaces against the prosthesis willensure that the prosthesis reversibly translates during articulation,maintaining the prosthesis, on average, in the same location for anygiven degree of knee articulation. The centered, rest position, of theprosthesis is usually determined when the knee is in extension and thereis maximum contact between the femoral condyle and the prosthesis. Inorder to ensure the ability of the prosthesis to “self-center,” adequatetension of the cruciate ligaments should be maintained.

While the prosthesis according to the present invention is shown anddescribed herein as being implanted in a knee joint, it is understoodthat the prosthesis could be utilized in joints other than the knee,such as the hip, shoulder, wrist, ankle, or elbow.

Turning now to FIGS. 1-4, a first embodiment of an implantable kneeprosthesis according to the present invention is illustrated anddesignated generally by reference numeral 100. Prosthesis 100 includes abody 102 having a generally elliptical shape in plan and including abottom, or tibial, surface 104 and an opposite top, or femoral, surface106. Bottom surface 104 is substantially flat, wherein the contour ofbottom surface 104 is preferably generally the same as the associatedcontour of the tibial plateau after any necessary bone resection asdescribed below. Due to its flat contour, bottom surface 104 isadvantageously optimized for use with tibial plateaus with large (>0.5mm) defects. When large tibial defects are present, potentialcomplications can include the femoral condyle disengaging from thefemoral surface of the prosthesis, traveling anteriorly or posteriorlyindependent of the prosthesis, and causing the prosthesis to disclocateor cause pain for the patient. Since flat bottom surface 104 is notsusceptible to catching on large tibial defects, prosthesis 100 extendsthe indications for patients requiring a knee prosthesis.

Top surface 106 is preferably generally concave, however it isunderstood that other contours of top surface 104 are fully contemplatedin accordance with the present invention. For example, for a medialcompartment implantation, top surface 106 preferably ranges fromgenerally flat to concave, whereas for a lateral compartmentimplantation, top surface 106 can range from generally convex togenerally concave depending on the condition of the ligaments and othersoft tissue structure at the time of surgery and how much stability theknee will require. It is understood that the terms “concave” and“convex” as used herein is not restricted to describing a surface with aconstant radius of curvature, but rather is used to denote the generalappearance of the surface. FIG. 9 shows an anterior/posteriorcross-sectional view of prosthesis 100 taken along section line A-A ofFIG. 2. Similarly, FIG. 10 illustrates a medial/lateral cross-sectionalview of prosthesis 100 taken along section line B-B of FIG. 2.

With continued reference to FIGS. 1-4, body 102 further includes aperipheral edge 112 extending between bottom surface 104 and top surface106 and having a first side 114, a second side 116 opposite first side114, a first end 118, and a second end 120 opposite first end 118. Asshown, edges along the periphery of prosthesis 100 are rounded ratherthan presenting sharp corners, such as in those devices of U.S. Pat. No.5,158,574. This rounded periphery is desired due to the fact thatprosthesis 100 is preferably allowed to move within the joint cavity.Periphery 108 of body 102 is on average of greater thickness than acentral portion 110 of body 102 (see FIG. 1), and preferably body 102generally has a negative meniscus shape when viewed from the side (seeFIG. 3) or in a section across body 102 in an anterior-posteriordirection from first end 118 to second end 120 (see, for example, FIG.9). Furthermore, in a preferred embodiment, a section across the body ina medial-lateral direction from first side 114 to second side 116 has athickness at a periphery of first side 114 which is larger on averagethan a thickness at a periphery of second side 116 (see FIG. 10).

As best shown in FIGS. 1-2, in the first embodiment of prosthesis 100,second side 116 of body 102 includes a pair of lobes 122 a and 122 b,where lobe 122 a is adjacent first end 118 and lobe 122 b is adjacentsecond end 120 with an indentation 124 formed therebetween. Whenimplanted in a patient's knee compartment (see FIG. 12), indentation 124will be proximate to the tibial spine and can preferably be designed tovary in shape from patient to patient as necessary due to the greatrange of variability of human anatomy. With indentation 124, prosthesis100 is generally kidney-shaped when viewed in plan, with the shaperesembling a distorted ellipse.

With reference to FIGS. 5-8, a second embodiment of prosthesis 100′ isdepicted, wherein reference numerals are the same as the embodiment ofFIGS. 1-4 except for the addition of a prime (′) designation. If thetibial spine has been substantially resected, no side indentation isnecessary and prosthesis 100′ is preferably used as it has a generallystraight second side 116′ without lobes 122 a, 122 b. With thisexception, it is understood that the foregoing description of prosthesis100 applies equally well to prosthesis 100′. Accordingly, it isunderstood that the term “generally elliptical” is intended to includeall construction methods which yield a generally planar shape which islonger in one direction than in the transverse direction and has roundedcorners, and that prosthesis 100, 100′ of the present invention is nototherwise limited to any particular shape.

Contrary to most devices which are composed of soft, compliant materialdesigned to assume the function of the natural meniscus which theyreplace, prosthesis 100, 100′ of the present invention comprises arelatively hard, relatively high modulus, preferably low frictionmaterial. Suitable materials include, for example, metals such as steelor titanium, metal alloys such as those described in U.S. Pat. Nos.3,989,517; 5,368,659; and 5,618,359 (LiquidMetal, Inc.), ceramics,hydrogels, and reinforced and non-reinforced thermoset or thermoplasticpolymers. From the hardness desired of prosthesis 100, 100′ it isreadily apparent that the prosthesis functions in a manner completelydifferent from those of the prior art. It is understood that the term“hard” as used herein is used to describe a material that is sufficientto span defects in the tibia or femur without substantially deforminginto the defects, allowing for the provision of recessed ornon-contacting areas of the prosthesis to encourage articularregeneration. Since the naturally occurring articular and meniscalmaterials found in the human knee joint demonstrate and possess uniquevisco-elastic properties not yet available in man-made materialformulations, if such a material were to become available, it would mostlikely represent the ideal material composition for prosthesis 100,100′.

Prosthesis 100, 100′ need not be made only of a single material, butcomposite structures may be used. In greater detail, materials couldinclude elastomeric polymers such as nylon, silicone, polyurethane,polypropylene, polyester, or the like, optionally fiber-reinforced, orviscous-elastic materials such as hydrogels, as well as otherhydrophilic materials or hydrophobic materials. Materials, such asbiocompatible polymers, capable of containing living cells could also beutilized. Still other possible materials are those which can replicatethe function of naturally occurring cartilage or meniscus such as theCSTI meniscal repair material that is the subject of numerous U.S.patents to Stone, for example, U.S. Pat. No. 5,158,574. A surfacecoating can be employed, such as for the reduction of friction ofprosthesis 100, 100′. Generally, the areas of prosthesis 100 expected tohave the most wear due to either high stress or greater movementrelative to the femoral condyle or tibial plateau may be made ofstronger, more abrasion resistant material than the remainder ofprosthesis 100, 100′ when composite structures are used. However, it isunderstood that any single component may be softer than the materialused for constructing the majority of prosthesis 100, 100′.

In accordance with the present invention, prosthesis 100, 100′ may bemanufactured so as to substantially contain, or have deposited thereon,a biologically or pharmaceutically active material such as, for example,one that promotes tissue regrowth, retards tissue degeneration, ordecreases inflammation. This is particularly suitable when prosthesis100, 100′ functions to bridge a defective area of bone or articularcartilage. The active material can be provided in the form of a coatingon prosthesis 100, 100′, or can be contained within prosthesis 100, 100′in the form of a solid, liquid, gel, paste, or soft polymer material.Such active materials may be designed to be delivered at once or in atimed-release manner. Preferably, the area of prosthesis 100, 100′containing the active material does not actually contact, or minimallycontacts, the damaged tissue.

As stated above, one purpose of the prosthesis 100, 100′ of the presentinvention is to achieve a span-like effect to bridge areas of thefemoral condyle and/or tibial plateau which have been damaged or haveexperienced tissue degeneration. If too soft and/or low modulus of amaterial were to be used for the prosthesis as in prior art devices, notonly would the load not be concentrated on healthy tissue, but damagedareas would also be subjected to static and dynamic loading and wear,thereby decreasing the opportunity for the body's natural regenerativecapability to function. Under such circumstances, active materials willbe rapidly dissipated and newly regenerated articular cartilage nothaving the necessary density or cohesiveness to withstand wear willquickly be eroded away. Rather than substantially deforming as in priorart devices to distribute a load relatively equally on the matingfemoral and tibial surfaces, prosthesis 100, 100′ according to thepresent invention does not necessarily spread the load uniformly, butrather may redistribute the load to healthy tissue, spanning areas ofimperfection and allowing inflamed, diseased, or other damaged areas toregenerate. Moreover, as regeneration proceeds, the regenerating tissuewill assume a shape dictated by the shape of prosthesis 100, 100′.Growth under these circumstances has the greatest potential for dense,ordered cartilage most closely replicating the original surface.

Much study has been dedicated to determine if any relationship exists inthe normal human anatomy that would allow one to define the requireddimensions of the prosthesis for proper fit and function based on asingle, easy to establish, measurable anatomic landmark. Based on astudy of over 100 MRI's and 75 X-rays of human subjects ranging from 15to 87 years of age, a relationship was established between theanteroposterior radius of the most distal portion of the femoral condyleand the dimensions which control the geometric form of the prosthesis.The database revealed a range of femoral anteroposterior radii from 32mm to 48 mm. However, it is known that the worldwide range is muchlarger because of genetic differences in the human anatomy.

With reference now to FIG. 11, a preferred method of construction of theprosthesis of the present invention aligns the apex of a femoral radiuswith the Coordinate System Origin (CSO) 10. While prosthesis 100 isshown and described, it is understood that the following discussion alsoapplies to prosthesis 100′. The apex of a tibial surface is alsogenerally aligned in both the anterior/posterior with the CSO 10, but isseparated vertically from the CSO 10 to create the part thickness. Thesubstantially elliptical shape of peripheral edge 112 is then locatedwith respect to the CSO 10. In general, the CSO 10 of the prosthesis 100is located at the center of the ellipse. It has been found that asuitable size for body 102 is defined by a minor axis of the ellipse F(defined by first side 114 and second side 116) and a major axis D(defined by first end 118 and second end 120) which are related by aratio ranging from F=0.25D to 1.5D. Similar ratios can be establishedfor all of the controlling dimensions of the part such that the shape inplan, femoral surface geometry, and tibial surface geometry for a normaltibial anatomy can generally be defined by one physicalanterior/posterior measurement of the patient's tibial anatomy. Theappropriate thickness of the prosthesis 100 can be determined bymeasuring the amount of joint space between the femoral and tibialsurface when a minor amount of valgus (heels out, knees in) is appliedto the knee.

Referring to FIGS. 9-11, the preferred relationship between femoralradius RA (see FIG. 9) to other joint dimensions (femoral radius is thedriving relationship to all other dimensions) is as follows:

-   -   Medial-lateral radius RB=0.25RA to 1.0RA (see FIG. 10)    -   Curve of anterior half of femoral radius RC=0.5RA to 2.0RA,        posterior half is straight    -   Length D=0.6RA to 1.4RA    -   Posterior half E=0.1RA to 0.75RA    -   Width F=0.25RA to 1.5RA    -   Width from part center to medial edge G=0.096RA to 0.48RA    -   Anterior plan radius RH=0.16RA to 0.64RA    -   Posterior plan radius RM=0.16RA to 0.64RA    -   Radius along lateral spine area RP=0.1RA to 2.0RA    -   Width from part center to lateral edge Q=−0.32RA to 0.32RA    -   Location of transition from anterior radius to medial radius        Y=−0.32RA to 0.32RA

(A negative value means that a dimension may extend to an opposite sideof section line A-A).

The actual shape of prosthesis 100, 100′ may be tailored to theindividual. Individuals with high varus or valgus deformation due towear, degeneration, or disease may require a prosthesis which is ofconsiderably greater thickness over the portions where wear is mostadvanced. In youthful patients, where trauma-induced damage rather thansevere wear or degeneration has occurred, differences in prosthesisthickness will be more moderate.

The axis of rotation of the tibia on the femur is 90 degrees to the pathof the tibial plateau against the femoral condyle. The two tibialplateaus (medial and lateral) are not in the same plane with each otherbut do act in a relatively constant radius to their respective femoralcondyles. In other words, although the symmetry of the femoral side ofthe prosthesis may be matched with the femoral condyle while the leg isin full extension, the rotation of the tibial plateau against thefemoral condyle is along a constant axis of rotation (90 degrees to theaxis of rotation), thus the angularity of the axis of symmetry of thefemoral condyle relative to the axis of symmetry of the tibial plateauis not parallel but at some acute angle. Also, the axis of symmetry ofthe tibial plateau is not parallel to the path of rotation of the tibiarelative to the femur, but also at some mildly acute angle. Thus, thetrue orientation of the prosthesis, regardless of the relativeorientations of symmetry of the tibial side to the femoral side is 90degrees to the true axis of rotation as described in Hollister et al.,“The Axes of Rotation of the Knee,” Clin Orthopaedics and Rel Res 290,pp. 259-268, 1993, herein incorporated by reference. Any localizedpositions of higher loads are self-limiting due to the ability of theprosthesis to translate both rotationally and laterally which mimics thetrue motion of the natural meniscus as described by Hollister.

During the load bearing portion of the gait cycle, or stance phase,flexion at the knee does not typically exceed 35°. Thus, the highestcompressive loads in the knee occur with the knee substantiallyextended. The outer contours of the prosthesis are preferably designedto substantially mate with the corresponding tibial and femoral surfaceswhen the knee is in full extension so that the high compressive loadscan be distributed over large surface areas. The contact areas betweenthe femoral condyle and the femoral surface of the prosthesis andbetween the tibial plateau and the tibial surface of the prosthesis aresubstantially equivalent during extension. However, because the contourof the femoral surface is more concave, the femoral condyle determinesthe position of the prosthesis on the surface of the tibial plateau inextension.

As the knee is flexed, the mating along the tibial surface issubstantially maintained. However, the contoured mating surfaces of thefemoral condyle and femoral surfaces of the prosthesis of the presentinvention can become increasingly dissimilar when the joint articulates.As the knee is flexed, there is a relative external rotation andposterior translation of the femur with respect to the tibia. Thus, thecontour angle of the femur becomes more in-line with the contour angleof the tibia in flexion. This can cause relative lateral or rotationalmovement, in the tibial plane, between the femoral condyle and thefemoral surface of the prosthesis. The forces generated by theincreasingly different geometry creates a rotational moment in thetibial plane which is resisted along the mating tibial surfaces andwhich also results in a restoring force tending to correctly locate theprosthesis along the femoral condyle. Thus, the prosthesis isself-centering to the femoral condyle, in part as a result of theconformity between the femoral condyle and the femoral surface of theprosthesis.

By changing the femoral surface of the prosthesis, it is possible toreduce the rotational moment induced during flexion by the mismatchbetween the femoral surface of the prosthesis and the femoral condyle. Apreferred method to accommodate this motion is to have a less acutealignment between the femoral and tibial axes of symmetry posterior tothe anterior/posterior midline, thereby reducing the mismatch betweenthe two axes in flexion. This angle is preferably 0° and can range from+/−15°. Anterior to the midline, the femoral contour is bent around aradius RC that is tangent to the posterior section of the sweep plane atthe most distal point of the femoral anterior/posterior radius RA. Thisfemoral surface geometry is essentially a compromise between thedifferent extension and flexion alignments of the femoral and tibialaxes of symmetry.

Because prosthesis 100, 100′ according to the present inventionpreferably has no physical method of attachment, the generally concavefemoral surface serves to locate the prosthesis through all ranges ofmotion, provided that the collateral ligaments are in proper tension. Bythe very nature of the ability to adjust for the lost articular materialthrough the thickness of the prosthesis, the thickness adjustmentsubstantially eliminates the need for a functional meniscus as a bearingsurface in a severely (Grade III or IV) degenerated knee. In theseinstances, the femoral surface of the prosthesis resides significantlyabove the meniscal edge, and the meniscus is completely unloaded.Prosthesis 100, 100′ according to the present invention also increasesthe translational stability of the knee, as the conforming shape of thefemoral surface plus re-tensioning of the collateral ligaments limitsexcessive anterior to posterior translation of the femur.

In some cases is may be necessary to add “reverse (downward)” curves, orcusps 132, to prosthesis 100, 100′ as shown in FIG. 13, located alongthe lateral aspect (of a medial device) of the device at the extremeanterior/lateral and posterior/lateral protrusions. Such circumstanceswould be when there is deformed anatomy or additional stabilization isrequired of prosthesis 100, 100′.

Generally speaking, each knee presents a different geometry of therespective femoral condyles and tibial plateaus. Even with respect tothe right and left knees of a single individual, although bilateralsymmetry dictates that the left and right knee components should bemirror images, this is often only an approximation. Thus, the shape ofthe affected femoral condyle and tibial plateau (while discussed hereinin the singular, more than one pair of condyle(s)/plateau(s) may beinvolved), will have to be ascertained to determine the correct geometryof the prosthesis 100 for a given patient.

To implant a prosthesis that possesses the characteristics required bythe subject invention, the patient's knee joint may be examined by anon-invasive imaging procedure capable of generating sufficientinformation such that one appropriately sized and shaped device may beselected. A variety of non-invasive imaging devices may be suitable, forexample magnetic resonance imaging (MRI), X-ray devices and the like.

Two methods of non-invasive imaging for selection of a suitableprosthesis 100, 100′ are preferred. In the first method, MRI or othernon-invasive imaging scans, optionally coupled with exteriormeasurements of the dimensions of the relevant tibial and femoralportions including the surface of the articular cartilage of the tibiaand femur, may be used to establish a library of prostheses whose sizeand geometry differ according to the age and size of the patient, thepatient's genetic make-up, and the like. A limited number of “standard”prostheses are then made to meet the requirements of a genericpopulation of patients. In this first method, a non-invasive imagingscan, such as an X-ray or MRI, together with knowledge of the patient'sgenetic make-up, general body type, extent of the disease, degeneration,or trauma and the like, will enable the surgeon to select a prosthesis100, 100′ of the correct size and shape from the library for thepatient.

In a second method, each patient receives one or more prostheses thatare custom tailored for the individual by producing a contour plot ofthe femoral and tibial mating surfaces and the size of the meniscalcavity. Such a contour plot may be constructed from imaging data (i.e.,X-ray or MRI data) by a suitable computer program. From the contourplot, the correct surface geometry of the prosthesis 100, 100′ isdetermined from the shape of the respective tibial plateau and femoralcondyle and the orientation between the two surfaces in extension. Ingeneral, the shapes just mentioned also include the articular cartilagewhich is typically maintained substantially intact.

In accordance with the present invention, it has been discovered thatthe amount of varus deformity is the primary, non-invasive method fordetermining the necessary thickness of the prosthesis 100 required forproper functioning. Viewing a weight bearing anteroposterior X-ray, acut and paste of the line drawn through the femoral condyles andrepositioned to put them once again parallel to the tibial plateaus willyield a measurement for the approximate thickness of the prosthesis.However, typically the proper thickness of the prosthesis is determinedintra-operatively.

The prosthesis 100, 100′ is introduced by arthroscopically assistedimplantation, typically via a 3 cm to 5 cm medial parapatella incision.The natural meniscus may be maintained in position or may be wholly orpartially removed, depending upon its condition. Under ordinarycircumstances, pieces of the natural meniscus which have been torn awayare removed, and damaged areas may be trimmed as necessary. In somewhatrarer instances, the entire portion of the meniscus residing in themeniscal cavity may be removed or is not present. If significant tibialbone loss (>0.5 mm) is found to be present, the area medial to thetibial spine may be flattened, or more of the surrounding cartilage andsubchondral bone removed to provide a stable, relatively flat tibialsurface. In some cases, it will be found necessary to remove all or aportion of the tibial spine as well. The tibial plateau is resected to amore flat configuration by cutting, in a minimal fashion, along thesagittal plane, similar to the method used in a TKR or UKR, but only toa depth of less than 5 mm and preferably to a depth of only 1-3 mm,allowing prosthesis 100, 100′ to bridge the defect without concern forthe defect impeding the motion of the prosthesis. Advantageously, nomechanical fixation of prosthesis 100, 100′ is required.

Prosthesis 100, 100′ may also be used in conjunction with ACL or PCLrepair, tibial osteotomy or articular surfacing procedures such ascartilage transplantations or abrasion anthroplasty. Following insertionof the prosthesis, X-ray, fluoroscopy, or MRI may be used to assess thecorrect positioning of the prosthesis both intraoperatively as well aspostoperatively. Since the surgical procedures used are not severe, andalso not irreversible, an unsuitable prosthesis may be readily removedand replaced, either with a different prosthesis from the library, or bya custom prosthesis.

FIG. 10 illustrates prosthesis 100 positioned in a right knee joint 200between a femur 202, including the femoral condyles 204, and a tibia 206including the tibial plateau 208. The femur 202 and tibia 206 includeinterconnecting collateral ligaments 210. FIG. 10 illustrates theposition of first side 114, second side 116, first end 118, and secondend 120 of prosthesis 100 when inserted in the medial compartment of apatient' right knee joint 200. It is understood that prosthesis 100′would be similarly positioned in the knee joint 200, and that prosthesis100, 100′ may just as easily be implanted in a lateral compartment or inthe left knee of a patient.

Following any necessary bone resection, one preferred surgical procedurewhich may be used to implant prosthesis 100, 100′ according to thepresent invention includes the following steps:

1. Verify preoperative indications:

-   -   a. Valgus determination of <5° with erect anterior/posterior        X-ray;    -   b. Medial compartment disease only. Some lateral spurs may be        present; and    -   c. Pre-operative sizing via medial/lateral template measurement        of anterior/posterior X-ray.

2. Standard Arthroscopy surgical prep:

-   -   a. Infiltrate knee with Lidocaine/Marcaine and Epinephrine.

3. Arthroscopy:

-   -   a. Inspect lateral patello-femoral compartments for integrity,        some mild arthosis is acceptable;    -   b. Removal of medial meniscus toward the rim along the anterior,        medial and posterior portions;    -   c. Initial arthroscopic osteophyte removal via ⅛″ osteotome and        burr to allow for valgus positioning of the knee;    -   d. Complete the removal (to the rim) of the posterior and        posterior-lateral meniscus; and    -   e. Confirm sizing of the prosthesis by measuring distance from        resected meniscus to remaining anterior meniscus.

4. Medial parapatellar arthrotomy (mid-patella to tibial joint line).

5. Complete removal of visible osteophytes along the medial femoralcondyle.

6. Insert thickness gauge and size for thickness of prosthesis.

7. Insert trial component:

-   -   a. Flex knee to approximately 50+ degrees to fully expose the        distal portion of the femoral condyle;    -   b. Insert trial component; and    -   c. While applying insertion pressure, apply valgus stress to the        tibia and “stretch-extend” the tibia over the trial component.

8. Check for proper sizing with “true lateral” and anterior/posteriorfluoroscope images of the knee while in extension:

-   -   a. Ideally, the prosthesis should be within 1 mm of the        anterior/posterior boundaries of the tibial plateau and        superimposed over the medial boundary.

9. Remove trial component and flush joint with saline.

10. Insert the appropriate prosthesis.

11. Confirm proper placement and sizing with fluoroscopic images as withtrial component.

12. Maintain leg in extension and close wound after insertion of aHemovac drain.

13. Place leg in immobilizer prior to patient transfer.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. The words used in the specification arewords of description rather than limitation, and it is understood thatvarious changes may be made without departing from the spirit and scopeof the invention.

1. A unicompartmental knee prosthesis for implantation into a knee jointcompartment between a femoral condyle and its corresponding tibialplateau, the prosthesis comprising: a body having a generally ellipticalshape in plan and having a top surface and an opposed, substantiallyflat bottom surface, the prosthesis configured to be translatable withrespect to the tibial plateau during knee articulation, the bodyincluding a peripheral edge extending between the top and bottomsurfaces and having a first side, a second side opposite the first side,a first end, and a second end opposite the first end, wherein the secondside comprises a straight segment extending from the first end to thesecond end.
 2. The prosthesis according to claim 1, wherein the topsurface is generally concave.
 3. The prosthesis according to claim 1,wherein the bottom surface includes a contour that is generally the sameas a corresponding contour of the tibial plateau following anyresection.
 4. The prosthesis according to claim 1, wherein the body isconstructed of a biocompatible material selected from the groupconsisting of ceramics, metals, metal alloys, hydrogels, reinforced andnon-reinforced thermoset or thermoplastic polymers, or compositesthereof.
 5. The prosthesis according to claim 1, wherein the bodyincludes at least one portion made of a relatively low modulus material.6. The prosthesis according to claim 5, wherein the relatively lowmodulus material is selected from the group consisting of reinforced andnon-reinforced elastomeric polymers, viscous-elastic materials, andhydrogels.
 7. The prosthesis according to claim 1, wherein the bodyincludes a material capable of containing living cells.
 8. Theprosthesis according to claim 1, further comprising a surface coatingapplied to the body.
 9. The prosthesis according to claim 1, wherein thebody includes an active material associated therewith.
 10. Theprosthesis according to claim 1, wherein the top surface and the bottomsurface are contoured such that the prosthesis is self-centering withinthe knee joint compartment.
 11. The prosthesis according to claim 1,wherein a first dimension D is defined by the first end and the secondend, and a second dimension F is defined by the first side and thesecond side, wherein the dimension F is from about 0.25D to about 1.5D.12. The prosthesis according to claim 1, wherein a section across thebody from the first end to the second end generally has the shape of anegative meniscus.
 13. The prosthesis according to claim 1, wherein asection across the body from the first side to the second side has athickness at a periphery of the first side which is larger on averagethan a thickness at a periphery of the second side.
 14. The prosthesisaccording to claim 1, wherein a periphery of the body is on average ofgreater thickness than a central portion of the body.
 15. The prosthesisaccording to claim 1, wherein the body comprises at least one cusp. 16.A unicompartmental knee prosthesis for implantation into a knee jointcompartment between a femoral condyle and its corresponding tibialplateau, the prosthesis comprising: a body having a generally ellipticalshape in plan and a substantially flat bottom surface and an opposed topsurface, the prosthesis configured to be translatable with respect tothe tibial plateau during knee articulation, the body including at leastone cusp for limiting translation of the prosthesis, the body includinga peripheral edge extending between the top and bottom surfaces andhaving a first side, a second side opposite the first side, a first end,and a second end opposite the first end, wherein the second sidecomprises a straight segment extending from the first end to the secondend.
 17. A method for implanting a unicompartmental knee prosthesis intoa knee joint compartment between a femoral condyle and its correspondingtibial plateau, the method comprising: providing a prosthesis includinga body having a generally elliptical shape in plan and having a topsurface and an opposed, substantially flat bottom surface, theprosthesis configured to be translatable with respect to the tibialplateau during knee articulation, the body including a peripheral edgeextending between the top and bottom surfaces and having a first side, asecond side opposite the first side, a first end, and a second endopposite the first end, wherein the second side comprises a straightsegment extending from the first end to the second end; surgicallyexposing the knee joint compartment; and inserting the prosthesis intothe knee joint compartment.
 18. The method according to claim 17,further comprising providing an active material associated with thebody.
 19. The method according to claim 17, further comprising applyinga surface coating to the body.
 20. The method according to claim 17,further comprising altering the condition of tissue located in the kneejoint compartment.
 21. The method according to claim 20, whereinaltering the condition of tissue includes resecting a portion of thetibial plateau.
 22. The method according to claim 20, wherein alteringthe condition of tissue includes resecting at least a portion of thetibial spine.
 23. The method according to claim 17, further comprisingdetermining a size and shape of the prosthesis required by examinationof the knee joint.
 24. The method according to claim 23, wherein theexamination includes one or more of X-ray imaging and MRI imaging. 25.The method according to claim 23, further comprising generating a customprosthesis whose size and shape are at least partially based on theexamination of the knee joint.
 26. The method according to claim 17,further comprising selecting the prosthesis from a library of prosthesesof standard shapes and sizes.
 27. The method according to claim 17,further comprising limiting translation of the prosthesis with at leastone cusp provided on the body.
 28. The method according to claim 17,wherein inserting the prosthesis includes inserting the prosthesis intoa medial compartment of the knee joint and moving an axis of rotation ofthe knee joint to a less varus condition.
 29. The method according toclaim 17, wherein inserting the prosthesis includes inserting theprosthesis into a lateral compartment of the knee joint and moving anaxis of rotation of the knee joint to a less valgus condition.
 30. Amethod for implanting a unicompartmental knee prosthesis into a kneejoint compartment between a femoral condyle and its corresponding tibialplateau, the method comprising: providing a prosthesis including a bodyhaving a generally elliptical shape in plan and having a top surface andan opposed, substantially flat bottom surface, the prosthesis configuredto be translatable with respect to the tibial plateau during kneearticulation, the body including at least one cusp for limitingtranslation of the prosthesis, the body including a peripheral edgeextending between the top and bottom surfaces and having a first side, asecond side opposite the first side, a first end, and a second endopposite the first end, wherein the second side comprises a straightsegment extending from the first end to the second end; surgicallyexposing the knee joint compartment; and inserting the prosthesis intothe knee joint compartment.